Skeletal muscle differentiation is a highly coordinated multi-step process in which mononucleated myoblasts first withdraw from the cell cycle upon extracelluar cues, differentiate into post-mitotic myocytes (early differentiation), and subsequently fuse into multi-nucleated myotubes (late differentiation), which finally bundle to form mature muscle fibers (terminal differentiation). This process is elaborately controlled by the activation of myogenic factor 5 (Myf5), myogenic differentiation antigen (MyoD), myogenin, and muscle regulatory factor 4 (MRF4): four myogenic regulatory factors (MRFs) belonging to a family of basic helix-loop-helix transcription factors. During myogenesis, MRFs are activated and operate in concert with other transcriptional regulators, such as myocyte enhancer factor 2 (MEF2), in a space- and time-correlated manner to regulate the transcription of muscle-specific genes including myosin heavy chain (MyHC) and muscle creatine kinase (MCK) (Braun et al., Nat. Rev. Mol. Cell. Bio. 12:349-361, 2011; Molkentin et al., Cell 83:1125-1136, 1995; Olson et al., Genes Dev. 8:1-8, 1994). Previous studies have confirmed that Myf5 and MyoD are muscle determination factors which are mainly expressed in undifferentiated myoblasts and differentiating myocytes, while myogenin is activated at an early stage of differentiation (Berkes et al., Semin. Cell Dev. Biol. 16:585-595, 2005). MRF4 has been shown to be transiently expressed during somitogenesis and later fiber maturation (Hinterberger et al., Dev. Biol. 147:144-156, 1991), and to play a role in myogenic lineage commitment (Kassar-Duchossoy et al., Nature 431:466-471, 2004), as well as myoblast fusion and differentiation (Suelves et al., EMBO J. 23:365-375, 2004; Sumariwalla et al., Genesis 30:239-249, 2001).
Muscle disorders, such as muscle atrophy, muscle weakness, myopathy, chronic fatigue syndrome, fibromyalgia, muscular dystrophy, fatigue fibromyalgia, spinal muscle atrophy, distal muscular dystrophy, dermatomyositis, polymyositis, rhabdomyolysis, polymyalgia rheumatica, and claudication, are characterized by a loss in muscle fiber function or mass in a mammal. Muscle disorders affect a significant population of humans worldwide. For example, Duschenne Muscular Dystrophy (DMD) occurs in 1 out of 3000 males in the U.S. (McPhee et al., Pathophysiology of Disease, Prentice Hall, 1995).